Dagmar Halbritter

Monocyte-Derived Human Macrophages Mediate Anergy in Allogeneic T Cells and Induce Regulatory T Cells

Activation of alloreactive T cells by APCs such as dendritic cells (DC) has been implicated as crucial step in transplant rejection. In contrast, it has been proposed that macrophages (Mφ) maintain tolerance toward alloantigens. It was therefore the aim of this study to further analyze the T cell-stimulatory capacity of mature DC and Mφ in vitro using the model of allogeneic MLR. There was a strong proliferative response in T cells cocultured with DC, which was further increased upon restimulation in a secondary MLR. In contrast, T cells did not proliferate in cocultures with Mφ despite costimulation with anti-CD28 and IL-2. Cytokine analysis revealed considerable levels of IL-10 in cocultures of T cells with Mφ, whereas high amounts of IL-2 and IFN-γ were present in cocultures with DC. There was only minimal T cell proliferation in a secondary MLR when T cells were rescued from primary MLR with Mφ and restimulated with DC of the same donor, or DC of an unrelated donor (third party), whereas a strong primary proliferative response was observed in resting T cells, demonstrating induction of T cell anergy by Mφ. Functional analysis of T cells rescued from cocultures with Mφ demonstrated that anergy was at least partly mediated by IL-10-producing regulatory T cells induced by Mφ. These results demonstrate that Mφ drive the differentiation of regulatory T cells and mediate anergy in allogeneic T cells, supporting the concept that Mφ maintain peripheral tolerance in vivo.

Mature But Not Immature Fas Ligand (CD95L)-Transduced Human Monocyte-Derived Dendritic Cells Are Protected from Fas-Mediated Apoptosis and Can Be Used as Killer APC

Several in vitro and animal studies have been performed to modulate the interaction of APCs and T cells by Fas (CD95/Apo-1) signaling to delete activated T cells in an Ag-specific manner. However, due to the difficulties in vector generation and low transduction frequencies, similar studies with primary human APC are still lacking. To evaluate whether Fas ligand (FasL/CD95L) expressing killer APC could be generated from primary human APC, monocyte-derived dendritic cells (DC) were transduced using the inducible Cre/Loxp adenovirus vector system. Combined transduction of DC by AdLoxpFasL and AxCANCre, but not single transduction with these vectors, resulted in dose- and time-dependent expression of FasL in >70% of mature DC (mDC), whereas <20% of immature DC (iDC) expressed FasL. In addition, transduction by AdLoxpFasL and AxCANCre induced apoptosis in >80% of iDC, whereas FasL-expressing mDC were protected from FasL/Fas (CD95/Apo-1)-mediated apoptosis despite coexpression of Fas. FasL-expressing mDC eliminated Fas+ Jurkat T cells as well as activated primary T cells by apoptosis, whereas nonactivated primary T cells were not deleted. Induction of apoptosis in Fas+ target cells required expression of FasL in DC and cell-to-cell contact between effector and target cell, and was not dependent on soluble FasL. Induction of apoptosis in Fas+ target cells required expression of FasL in DC, cell-to-cell contact between effector and target cell, and was not dependent on soluble FasL. The present results demonstrate that FasL-expressing killer APC can be generated from human monocyte-derived mDC using adenoviral gene transfer. Our results support the strategy to use killer APCs as immunomodulatory cells for the treatment of autoimmune disease and allograft rejection.

Elimination of activated but not resting primary human CD4+ and CD8+ T cells by Fas ligand (FasL/CD95L)-expressing Killer-dendritic cells.

Dendritic cells (DC) genetically engineered to express high levels of Fas ligand (FasL/CD95L) have been demonstrated to delete T cells in an antigen specific manner in several different animal models in vivo. However, the immunomodulatory capacity of primary human FasL-expressing Killer-DC has not been determined. Therefore, human Killer-DC were generated from mature monocyte-derived DC using the inducible CRE/LoxP adenoviral vector system, and the immunoregulatory capacity of these cells was analyzed in cocultures with primary human T cells in vitro. Combined transductions of DC by AdloxPFasL and AxCANCre resulted in FasL expression in > 70% of DC without affecting the mature phenotype. Proliferation of activated primary human T cells was inhibited up to 80% in cocultures with FasL-expressing DC but not EGFP-transduced DC, which was due to induction of apoptosis in activated but not resting CD4+ and CD8+ T cells. Apoptosis induced by Killer-DC could be blocked by an anti-FasL-antibody in a dose dependent fashion. The present results demonstrate that FasL-expressing Killer-DC eliminate activated but not resting primary human CD4+ and CD8+ T cells by induction of Fas-mediated apoptosis supporting the concept to apply Killer-DC as a novel strategy for the treatment of T cell-dependent autoimmune disease and allograft rejection in humans.

Peripheral Blood Mononuclear Cells: Isolation, Freezing, Thawing, and Culture.

The work with peripheral blood mononuclear cells (PBMCs), which comprise lymphocytes and monocytes, is indispensable in immunological diagnostics and research. The isolation of PBMCs takes advantage of differences in cell density of the different blood components. Density gradient centrifugation of diluted whole blood layered over a density gradient medium yields PBMCs; two subsequent washing steps remove remaining platelets. To store the cells for future assays, they can be frozen and thawed when required. Dimethyl sulfoxide (DMSO) serves as a cryoprotectant for freezing PBMCs, but must be removed by washing after thawing, as it can become toxic to the cells on longer exposure.

A new flow cytometric assay for the simultaneous analysis of antigen-specific elimination of T cells in heterogenous T cell populations

Novel immunosuppressive strategies are targeting for an antigen-specific deletion of T cells responsible for organ damage in autoimmunity and allograft rejection. Here, we present a new flow cytometry-based assay that allows the reliable and efficient detection of T cells that were eliminated in an antigen-specific fashion. A stable cell-labelling technique utilizing the two membrane dyes PKH26 and PKH67 has been combined with annexin V and 7-aminoactinomycin (7-AAD) staining to detect apoptotic cells. A differential gating strategy enabled us to determine the viability/apoptosis for each PKH-stained T cell subpopulation independently. The capability to simultaneously analyze apoptosis within T cell mixtures of different antigen specificities establishes this assay as a superior tool for the further development of novel antigen-specific immunosuppressive approaches.

IL-3 promotes the development of experimental autoimmune encephalitis

Little is known about the role of IL-3 in multiple sclerosis (MS) in humans and in experimental autoimmune encephalomyelitis (EAE). Using myelin oligodendrocyte glycoprotein (MOG) peptide-induced EAE, we show that CD4+ T cells are the main source of IL-3 and that cerebral IL-3 expression correlates with the influx of T cells into the brain. Blockade of IL-3 with monoclonal antibodies, analysis of IL-3 deficient mice, and adoptive transfer of leukocytes demonstrate that IL-3 plays an important role for development of clinical symptoms of EAE, for migration of leukocytes into the brain, and for cerebral expression of adhesion molecules and chemokines. In contrast, injection of recombinant IL-3 exacerbates EAE symptoms and cerebral inflammation. In patients with relapsing-remitting MS (RRMS), IL-3 expression by T cells is markedly upregulated during episodes of relapse. Our data indicate that IL-3 plays an important role in EAE and may represent a new target for treatment of MS.

Alloantigen specific deletion of primary human T cells by Fas ligand (CD95L)-transduced monocyte-derived killer-dendritic cells

Numerous studies have been performed in vitro and in various animal models to modulate the interaction of dendritic cells (DC) and T cells by Fas (CD95/Apo‐1) signalling to delete activated T cells via induction of activation‐induced cell death (AICD). Previously, we could demonstrate that Fas ligand (FasL/CD95L)‐expressing ‘killer‐antigen‐presenting cells’ can be generated from human monocyte‐derived mature DC (mDC) using adenoviral gene transfer. To evaluate whether these FasL‐expressing mDC (mDC‐FasL) could eliminate alloreactive primary human T cells in vitro, co‐culture experiments were performed. Proliferation of human T cells was markedly reduced in primary co‐cultures with allogeneic mDC‐FasL, whereas a strong proliferative T‐cell response could be observed in co‐cultures with enhanced green fluorescent protein‐transduced mDC. Inhibition of T‐cell proliferation was related to the transduction efficiency, and the numbers of mDC‐FasL present in co‐cultures. In addition, proliferation of pre‐activated alloreactive CD4+ and CD8+ T cells could be almost completely inhibited in secondary co‐cultures using mDC‐FasL as stimulatory cells, which was the result of induction of apoptosis in the majority of preactivated T cells. The specific deletion of alloreactive T cells by mDC‐FasL was confirmed by an unaffected proliferative response of surviving T cells towards allogeneic ‘third‐party’ peripheral blood mononuclear cells in a third stimulation, or upon unspecific stimulation with anti‐CD3/CD28 beads. The results of this study demonstrate that allospecifically activated T cells are efficiently eliminated by mDC‐FasL, supporting further investigations to apply FasL‐expressing ‘killer‐DC’ as a novel strategy for the treatment of allograft rejection.

Expression of IL-3 receptors and impact of IL-3 on human T and B cells

A large number of animal models revealed that IL-3 plays an important role for the development of T and B cell-mediated autoimmune diseases. However, little is known about the expression and regulation of IL-3 receptors in human T and B cells and how IL-3 modulates the activation and survival of these cells. We show that the IL-3 receptor CD123 is substantially upregulated on proliferating CD4+ and CD8+ T as well as B cells. Upregulation of CD123 differs between various activators and can be further modulated by cytokines. Exposure of human T and B cells to IL-3 enhances proliferation and survival. IL-3 also induces a shift towards secretion of proinflammatory cytokines in T and B cells and reduces the expression of IL-10 in B cells. Thus IL-3 may have proinflammatory and immunostimulatory properties also in human autoimmune diseases.